This one is a much longer one, but it is worth reading and very much blew my tiny mind when I came across it, it is from the conferences of Cassian, his interview with Moses the Black:
All the arts and sciences, said he, have some goal or mark; and end or aim of their own, on which the diligent pursuer of each art has his eye, and so endures all sorts of toils and dangers and losses, cheerfully and with equanimity, e.g., the farmer, shunning neither at one time the scorching heat of the sun, nor at another the frost and cold, cleaves the earth unweariedly, and again and again subjects the clods of his field to his ploughshare, while he keeps before him his goal; viz., by diligent labour to break it up small like fine sand, and to clear it of all briers, and free it from all weeds, as he believes that in no other way can he gain his ultimate end, which is to secure a good harvest, and a large crop; on which he can either live himself free from care, or can increase his possessions. Again, when his barn is well stocked he is quite ready to empty it, and with incessant labour to commit the seed to the crumbling furrow, thinking nothing of the present lessening of his stores in view of the future harvest. Those men too who are engaged in mercantile pursuits, have no dread of the uncertainties and chances of the ocean, and fear no risks, while an eager hope urges them forward to their aim of gain. Moreover those who are inflamed with the ambition of military life, while they look forward to their aim of honours and power take no notice of danger and destruction in their wanderings, and are not crushed by present losses and wars, while they are eager to obtain the end of some honour held out to them. And our profession too has its own goal and end, for which we undergo all sorts of toils not merely without weariness but actually with delight; on account of which the want of food in fasting is no trial to us, the weariness of our vigils becomes a delight; reading and constant meditation on the Scriptures does not pall upon us; and further incessant toil, and self-denial, and the privation of all things, and the horrors also of this vast desert have no terrors for us. And doubtless for this it was that you yourselves despised the love of kinsfolk, and scorned your fatherland, and the delights of this world, and passed through so many countries, in order that you might come to us, plain and simple folk as we are, living in this wretched state in the desert. Wherefore, said he, answer and tell me what is the goal and end, which incite you to endure all these things so cheerfully.
And when he insisted on eliciting an opinion from us on this question, we replied that we endured all this for the sake of the kingdom of heaven.
To which he replied: Good, you have spoken cleverly of the (ultimate) end. But what should be our (immediate) goal or mark, by constantly sticking close to which we can gain our end, you ought first to know. And when we frankly confessed our ignorance, he proceeded: The first thing, as I said, in all the arts and sciences is to have some goal, i.e., a mark for the mind, and constant mental purpose, for unless a man keeps this before him with all diligence and persistence, he will never succeed in arriving at the ultimate aim and the gain which he desires. For, as I said, the farmer who has for his aim to live free from care and with plenty, while his crops are springing has this as his immediate object and goal; viz., to keep his field clear from all brambles, and weeds, and does not fancy that he can otherwise ensure wealth and a peaceful end, unless he first secures by some plan of work and hope that which he is anxious to obtain. The business man too does not lay aside the desire of procuring wares, by means of which he may more profitably amass riches, because he would desire gain to no purpose, unless he chose the road which leads to it: and those men who are anxious to be decorated with the honours of this world, first make up their minds to what duties and conditions they must devote themselves, that in the regular course of hope they may succeed in gaining the honours they desire. And so the end of our way of life is indeed the kingdom of God. But what is the (immediate) goal you must earnestly ask, for if it is not in the same way discovered by us, we shall strive and wear ourselves out to no purpose, because a man who is travelling in a wrong direction, has all the trouble and gets none of the good of his journey. And when we stood gaping at this remark, the old man proceeded: The end of our profession indeed, as I said, is the kingdom of God or the kingdom of heaven: but the immediate aim or goal, is purity of heart, without which no one can gain that end: fixing our gaze then steadily on this goal, as if on a definite mark, let us direct our course as straight towards it as possible, and if our thoughts wander somewhat from this, let us revert to our gaze upon it, and check them accurately as by a sure standard, which will always bring back all our efforts to this one mark, and will show at once if our mind has wandered ever so little from the direction marked out for it.
As those, whose business it is to use weapons of war, whenever they want to show their skill in their art before a king of this world, try to shoot their arrows or darts into certain small targets which have the prizes painted on them; for they know that they cannot in any other way than by the line of their aim secure the end and the prize they hope for, which they will only then enjoy when they have been able to hit the mark set before them; but if it happens to be withdrawn from their sight, however much in their want of skill their aim may vainly deviate from the straight path, yet they cannot perceive that they have strayed from the direction of the intended straight line because they have no distinct mark to prove the skilfulness of their aim, or to show up its badness: and therefore while they shoot their missiles idly into space, they cannot see how they have gone wrong or how utterly at fault they are, since no mark is their accuser, showing how far they have gone astray from the right direction; nor can an unsteady look help them to correct and restore the straight line enjoined on them. So then the end indeed which we have set before us is, as the Apostle says, eternal life, as he declares, having indeed your fruit unto holiness, and the end eternal life; Romans 6:22 but the immediate goal is purity of heart, which he not unfairly terms sanctification, without which the afore-mentioned end cannot be gained; as if he had said in other words, having your immediate goal in purity of heart, but the end life eternal. Of which goal the same blessed Apostle teaches us, and significantly uses the very term, i.e., σκοπός, saying as follows, Forgetting those things which are behind and reaching forward to those that are before, I press toward the mark, for the prize of the high calling of the Lord: Philippians 3:13-14 which is more clearly put in Greek κατὰ σκοπὸν διώκω, i.e., I press toward the mark, as if he said, With this aim, with which I forget those things that are behind, i.e., the faults of earlier life, I strive to reach as the end the heavenly prize. Whatever then can help to guide us to this object; viz., purity of heart, we must follow with all our might, but whatever hinders us from it, we must shun as a dangerous and hurtful thing. For, for this we do and endure all things, for this we make light of our kinsfolk, our country, honours, riches, the delights of this world, and all kinds of pleasures, namely in order that we may retain a lasting purity of heart. And so when this object is set before us, we shall always direct our actions and thoughts straight towards the attainment of it; for if it be not constantly fixed before our eyes, it will not only make all our toils vain and useless, and force them to be endured to no purpose and without any reward, but it will also excite all kinds of thoughts opposed to one another. For the mind, which has no fixed point to which it may return, and on which it may chiefly fasten, is sure to rove about from hour to hour and minute to minute in all sorts of wandering thoughts, and from those things which come to it from outside, to be constantly changed into that state which first offers itself to it.